1. Field of the Invention
The present invention relates to a base station for use in mobile communication by mobile phones or the like, and particularly to a public base station of PHS.
2. Description of the Background Art
Cellular phones, PHS (Personal Handyphone System) and others have been known as mobile communication system performing wireless communication via a base station.
In the PHS, a radio signal is formed of frames divided into eight slots. Among the eight slots, first four slots form a down signal sent from a base station to a mobile terminal, and second four slots form an up signal sent from the mobile terminal to the base station. This manner has been referred to as “Time Division Multiple Access (TDMA)”.
In the conventional base station, the four slots are assigned to one control channel and three information channels. The control channel transfers information relating to a channel structure, system information and others from the base station to the mobile station, and also transfers control information required for calling and connection. The information channel is a bidirectional channel transferring user information such as call or conversation information and data transmission information. The information channel to be used among the plurality of information channels is determined by transmitting a control signal between the base station and the mobile station via the channel for control.
Assuming that C represents the control channel and T represents the information channel, the conventional base station basically has a structure of 1C3T. Thus, one slot is used for sending and receiving the control signal, and the other three slots are used primarily for sending and receiving the user information.
The conventional base stations for the PHS include such a kind of base station that uses all the slots as the call channels, and thus 4T-base stations are connected to increase the number of information channels. For example, Japanese Patent Laying-Open No. 2000-102056 has pointed out a problem of a failure in such a base station of 1C7T.
FIG. 7 is a schematic block diagram of a conventional base station of 1C7T.
Referring to FIG. 7, a base station P is formed of a base station A1 of 1C3T-type and a base station A2 of 4T-type, which are connected together by a connection cable B, and operates as a base station of 1C7T-type as a whole.
Base station A1 includes an antenna 110, a radio-frequency portion 112, a wireless control portion 114, a line control portion 116, an inter-base interface portion 118 and a central control portion 120.
Antenna 110 is used for sending and receiving a radio signal to and from a portable terminal. Radio-frequency portion 112 is provided for converting frequencies of send and receive signals. Wireless control portion 114 is provided for controlling wireless communication with portable terminals, and operates to modulate data such as conversation data in an appropriate modulation method, e.g., according to a PHS communication method and to demodulate received data modulated in such modulation method.
Line control portion 116 is connected to an I′ line (ISDN (Integrated Service Digital Network) line for PHS) serving as a cable network for controlling communication between the I′ line and central control portion 120. Inter-base interface portion 118 is connected to an inter-base interface portion of another base station, and provides an interface between base station A1 and another base station. More specifically, it operates to send and receive control signals and synchronizing signals to and from another base station. Central control portion 120 is provided for controlling the foregoing operations of the portions in base station A1. Since base station A2 has a structure similar to that of base station A1, description thereof is not repeated.
Base stations A1 and A2 are configured to allow switching between a function of a master base station and a function of a slave base station. Thus, base stations A1 and A2 can be switched between master and slave states, i.e., between a state of the master station and a state of the slave station in accordance with predetermined switching factors.
When a failure occurs in the master base station, the slave base station is switched to operate as a master base station so that a function of a normal operation can be maintained as long as possible. These are disclosed in the Japanese Patent Laying-Open No. 2000-102056.
As described above, it has been studied to provide base stations configured such that two or more base stations can be cascaded by a signal line to operate them as one base station as a whole.
When arranged individually, each base station operates with 1C3T, i.e., with one control channel and three information channels. When two base stations of the above type are arranged, three users can make calls per base station, and six information channels in total can be ensured.
If two or more base stations are cascaded, only one base station is required to operate as a master base station (i.e., base station on a master side) having a control channel, and other base station(s) can use all the channels as information channels. Thus, in the case of employing two cascaded base stations, the master base station operates with 1C3T, and the slave base station (i.e., base station on the slave side) operates with 4T so that 1C7T is achieved in total. In this case, seven information channels can be ensured, and thus can offer service to seven users within an area of the two base stations. Thus, the information channels can be increased by one from those provided by two independent base stations.
When the base station allocates the channels, it usually gives higher priorities to the channels of better conditions. More specifically, the base station performs carrier sense (measurement of an amount of interference) with each slot and each frequency for channel allocation. In the cascade base station formed of base stations CS1 and CS2 connected together, when it receives a request for link channel establishment from a mobile terminal, the base station allocates the best channel to the mobile terminal based on a result of the carrier sense of each of base stations CS1 and CS2 as well as a value of an electric power of reception of the link channel request message. For example, a channel causing minimum interference is allocated to the mobile terminal.
However, traffic may concentrate on the base station (master base station) sending a control channel CCH. It can be considered that concentration of the traffic occurs because the master base station provides the best conditions at the time of the channel allocation to the mobile terminal. For example, users of the mobile terminals located near the master base station are more than those located near the slave base station. In this case, the cascaded base stations cannot accommodate the user any more as a whole because the master base station operating control channel CCH, although good conditions are achieved on the side of the master base station.
The PHS system usually performs the communication of up to 32 kbps per channel, but faster communication has recently been required in mobile communication for the Internet browsing and others. For example, such service is already offered that allows communication at 64 kpbs, i.e., double the ordinary speed by using two channels of 32 kpbs, as is standardized by the PHS Internet Access Forum Standard (PIAFS).
The service of 64 kbps can be offered only when two empty information channels are present in one base station at the time of reception of a request from the mobile terminal. Therefore, even when two or more empty information channels are present in the two cascaded base stations as a whole, the master base stations cannot offer the requested service of 64 kbps, if only one empty channel is present in the master base station at that point in time.